Measurement of plasma density in the discharge maintained in a nonuniform gas flow by a powerful radiation of terahertz-band gyrotron

We performed measurements of plasma density in a "point-like" discharge, which is generated in a nonuniform flow of a gas (argon) under the action of high-power terahertz gyrotron radiation with a frequency of 0.67 THz, a power of 40 kW, and a pulse duration of 20 mu s. The nonuniform flow was produced by injecting the gas to the vacuum chamber through a small hole (0.14 mm in diameter) under a background pressure at a level of 0.01 Torr. The discharge developed and was localized only in a small region of space (about 1 mm) near the gas injection hole, where the pressure was high (close to the atmospheric one) and the breakdown conditions (the "point-like" discharge) were fulfilled. The density of electrons in the discharge was measured by observing the Stark broadening of the atomic radiation line H-alpha. of hydrogen (656.3 nm) which was present in the discharge as a minor admixture. The plasma density in the discharge was equal to about 2.10(16) cm(-3), which exceeds the value being critical for a frequency of 0.67 THz used to maintain the radiation discharge.